New motors work best. Once the rubber has gone hard, you can't really bring it back. You can pull them out and soak them in hot water, give them a clean and even try a bit of glycerine on them, but you're only postponing the inevitable. They might not even come out in one piece. If your windows are struggling then the motors are probably already a gonner. Worth noting that even in this very cold weather we're having, my windows are still whizzing up and down with ice on them thanks to the replacement mechanisms.
The window glass does run in the rubbers, sure but if you pull them out and operate them up and down, you might find them really slow despite not having the seals in there.
The motor has this little pinion gear that engages in the quadrant rack. Over the years the rack wears, the bush that the pinion shaft sits in also wears so that when you operate the motor there's so much play that it all binds up. Greasing doesn't change that I'm afraid.